The present invention relates to high efficiency solar cells, and particularly to a high efficiency tandem solar cell composed of a substantially single crystal heteroepitaxial multilayer film to form three photoactive junctions electrically connected in series, and to its method of manufacture.
Spacecraft power systems require a technology that provides high solar energy conversion efficiency at a high specific power (power/mass) and high resistance to radiation present in the space environment. Photovoltaic solar cell arrays have been used extensively in spacecraft power systems, but to date have been limited to specific powers in the range of about 300 watts/kg. NASA goals for space power arrays call for specific powers for the array in excess of 300 watts/kg to reduce the mass and drag of the spacecraft power system and thereby increase payload capacity. The individual cells used in such an array must have specific powers well in excess of 300 watts/kg if the resulting array goal is to be achieved, as the hardware of the array reduces the specific power by adding weight without increasing power output.
The use of I-III-VI.sub.2 semiconductor cells (i.e. CuInSe.sub.2) in photovoltaic solar cell arrays can offer significant advantages over silicon solar cells in meeting the aforementioned array requirements, including:
(1) The CuInSe.sub.2 cell can generally be made thinner than conventional silicon cells thereby offering the potential of an extremely high specific power.
(2) Radiation testing has shown (i) that the CuInSe.sub.2 cell is at about 50 times more resistant to 1 MeV protons than silicon cells, and (ii) that the cell also possesses an inherent tolerance to irradiation by 1 MeV electrons up to at least 2.times.10.sup.16 electrons/cm.sup.2. At this level, typical silicon cells are degraded by over 50%. Because of the radiation hardness of the CuInSe.sub.2 cell, reduced radiation shielding is required which results in an even higher specific power. By using the equivalent radiation shielding on CuInSe.sub.2 cells as used on silicon cells, a higher end of life efficiency can be achieved for CuInSe.sub.2 as well as a higher specific power.
(3) Annealing of the cell, after proton irradiation, at 200.degree. C. for six minutes restores the CuInSe.sub.2 cell to within 95% of its initial efficiency.
CuInSe.sub.2 (i.e. CIS) semiconductors suitable for solar cells are described by Mickelsen and Chen in U.S. Pat. No. Re. 31,968 and U.S. Pat. No. 4,523,051, both of which are hereby incorporated by reference. The disclosed CuInSe.sub.2 /(Cd,Zn)S cells were deposited on relatively thick substrates selected from polycrystalline alumina, glazed alumina, enameled steel, metal foils, and similar inert inorganic materials. Typically, the substrate had a thickness of at least about 25 mils and was preferably either 25 mil alumina or 60 mil soda lime glass. Cells of this type had efficiencies on the order of 10% AMO, but the specific power of the cells was reduced by the mass of the substrate. Thus, while the soda lime glass or alumina substrates were satisfactory for terrestrial applications, a much lighter substrate was required to meet the demands for modern space power applications.
U.S. Pat. No. 4,703,131 by Dursch describes an improved CuInSe.sub.2 solar cell having a specific power in excess of 400 watts/kg and comprising a (Cd,Zn)S/CuInSe.sub.2 thin film on a 5 mil titanium metal foil. The higher specific power results from a more efficient transducer and a much lighter substrate. To achieve the maximum efficiencies and specific powers, efforts have been made to produce tandem solar cells having a potential specific power of at least 1000 watts/kg and a conversion efficiency of better than 20%. For example, U.S. patent application Ser. No. 072,316 (now U.S. Pat. No. 4,795,501) describes a monolithic, heteroepitaxial, double junction, GaAlAs/CuInSe.sub.2, tandem solar cell having a conversion efficiency of about 25% or higher with a specific power of at least about 1000 watts/kg. In the cell, the lattice mismatch transition zone comprises an epitaxial layer of ZnSSe adjacent a p.sup.+ -minority carrier confinement layer of a CLEFT double heterostructure, single crystal GaAlAs upper cell followed by a CdZnSeS layer adjacent a CuInSe.sub.2 lower cell. The tandem cell combines an upper cell noted for its high electron mobility and high absorption coefficient with an inexpensive thin film lower cell noted for its high absorption coefficient and good electron mobility.
The present invention provides a monolithic tandem solar cell having a potential efficiency of at least about 30% AMO and a specific power potential of over 1000 watts/kg at the cell level (with coverglass), as well as superior radiation resistance. Such a cell is easier to manufacture than the double junction of my earlier patent.